The Days on Saturn Just Got Shorter

Saturn as seen by the Cassini spacecraft / NASA/JPL/Space Science Institute

By Janet Fang

28 Mar 2015, 23:50

A new calculation of Saturn’s rotation period cuts its day down by a few minutes. A single day on the sixth planet from the sun lasts 10 hours, 32 minutes, 45 seconds (plus or minus 46 seconds). The findings were published in Nature this week.

The amount of time it takes for Saturn to rotate once around its axis has been notoriously difficult to determine. For rocky planets like Earth and Mars, measuring the rotation is fairly straightforward: Just find a surface feature and wait for it to roll back into view. But gas giants covered with thick layers of clouds like Saturn and Jupiter don’t have solid objects on their surface for these sorts of observations. To make it even harder, Saturn’s magnetic pole is aligned with its rotational axis. That means the rotation period can’t be calculated using the magnetic field—unlike with Jupiter, which has a tilted magnetic field.

For decades, we thought it was 10 hours, 39 minutes, 22 seconds based on radiofrequency emissions estimated by Voyager 2 in the 1980s. But when Cassini (pictured right) arrived 30 years later to repeat those radio measurements, the rotation period appeared to be 8 minutes longer. Furthermore, other methods—such as using the planet’s wind field or cloud tracking—have resulted in an estimate of between 10 hours, 32 minutes and 10 hours, 47 minutes.

Now, a trio led by Ravit Helled of Tel Aviv University is trying something else. “We came up with an answer based on the shape and gravitational field of the planet,” Helled says in a statement. “We were able to look at the big picture, and harness the physical properties of the planet to determine its rotational period." Starting with the shape and gravity field, they then worked backwards.

When a planet spins, its shape distorts: It flattens out a bit, gaining a sort of “belly” at its middle, study co-author Yohai Kaspi from the Weizmann Institute of Science explains in a news release. The faster it spins, the more pronounced this belly becomes. And because it’s mostly a dense fluid of helium and hydrogen, the planet deforms more easily. This redistribution of the planet’s mass creates fluctuations in its gravitational field—which can be measured using data from Cassini as well as telescopes. Finally, the gravity field measurements could then be used to calculate the big unknown variable: the rotation that produced them.

To confirm these findings, the team used the same method to accurately recover the rotation period of Jupiter—which is already known to be 9 hours, 55 minutes.